\[1)\ Точка\ \text{C\ }лежит\ на\ отрезке\ AB:\]
\[AC = \frac{1}{3}AB;\]
\[AB = AC + BC\]
\[AB = \frac{1}{3}AB + BC\]
\[BC = \frac{2}{3}\text{AB}\]
\[AC\ :BC = 1\ :2.\]
\[Ответ:\ \ AC = \frac{1}{3}\text{AB.}\]
\[2)\ Точка\ \text{A\ }лежит\ на\ отрезке\ BC:\]
\[AC = AB;\]
\[BC = AB + AC = 2AB\]
\[AC\ :BC = 1\ :2.\]
\[Ответ:\ \ AC = AB.\]